Communication system



1951 H. s. e. COOPER ET AL 2,577,512

COMMUNICATION SYSTEM Filed March 22, 1945 m up 8 EMU www cc a 1 5 7 n T M .$.L. a v M H RM .l. u M' Tl.

2 V G Emi i1 F V w II. lllll lllll b a u W w A 7' TORNEV Patented Dec. 4, 1951 COMMUNICATION SYSTEM Harold S. G. Cooper, Cambridge, and Howard Louis Schultz, Wellesley, Mass., assignors, by mesne assignments, to the United States of America as represented by the Secretary of War Application March-22, 1945, Serial No. 584,237

3 Claims.

This invention relates to a communication system, and, more particularly, to radio transmitters of the pulse modulation type used in radio direction and range determining systems.

I Radio tubes ordinarily require a positive plate voltage of constant magnitude for operation. Batteries may be used for Smaller systems, but are not satisfactory for larger radio transmitters. 'An alternating current source is therefore ordinarily used, and this power'must be rectified and filtered before being applied to the tubes. These rectifiers and filtering components add additional weight and increase power consumption.

One of the objects of this invention is to provide a circuit for operating a transmitter from an alternating current source without rectification and filtering. Another object of this invention is to eliminate rectifying andfiltering components and circuits thereby reducing the weight and powerrequirements of the transmitter.

Other objects will be apparent to those'skilled in the art after a study of the following specification and drawing, in which:

Fig. l is a circuit diagram of a pulse modulated radio transmitter constructed in accordance with the principlesof this invention; and

Fig. 2 illustrates the phase relationship of voltage waveforms in the transmitter. I

Referring now more particularly to Fig. 1, there is shown a pulse modulation radio transmitter including radio frequency oscillator tube 3. Tube :3 is a triode containing anode 4, grid 5, and cathode 6.

Power for tube 3 is supplied by power transformer I which has its primary winding 8 connected to a source of alternating current. The top of secondary winding 9 is connected to anode 4 of tube 3 through resistor l and choke II.

The secondary winding 9 is variably tapped to ground, as shown at I3. A capacitor I2 is connected between the junction of resistor 10, choke II and ground. A potentiometer I4 is connected between grounded tap l3 and the lower end of secondary winding 9. Movable arm I4 of potentiometer I4 is connected through resistor l5 to grid 5 of tube, 3 to supply the necessary tube bias. Resistor I5 is an isolating resistor to prevent grid 5 from drawing excessive current when the lower terminal of secondary 9 becomes positive.

The electrodes of tube 3 are connected, as shown, by capacitors l1, I8, and and inductance l9 to provide a conventional Colpitts type oscillator. The output of this oscillator is ob- 2- tained from coil 2| inductively coupled to inductance I9.

To pulse modulate the oscillator tube 3, there is provided a gas-filled triode 23 including anode 24, grid .25, and cathode 26. The voltage for anode 24 is supplied by secondary winding 28 of transformer "I through resistor 21.

A transformer 30 has one terminal of its secondary winding connected to grid 25 of tube-23, the other secondary winding terminal being grounded. Transformer 30, like anode 24, derives its power from secondary winding 28 of transformer 1.

Connected between secondary winding 28 and transformer 30 is a well known Helmholtz phase shifting device comprising fixed coils 33 and 34, resistor 35, condenser 36, and movable coil 32. The axis of coil 32 is variable in relation to the axes of coils 33 and 34. This phase shifting circuit alters the phase of the voltage supplied to grid 25 with respect to the voltage supplied to anodes 4 and 24 of tubes'3 and 23, respectively. Connected across the anode circuit of tube 23 is a pulse-forming network 31 comprised of inductances and capacitances, as shown.

Cathode 26 of tube 23 is connected to ground through a resistor 39. The voltage developed across resistance 39 is applied through lead-40 to grid 5 of oscillator tube 3. The impedance of resistor 39 may be approximately that of the characteristic impedance of pulse-forming line 31, since the impedance of tube 3 is high.

The arrangement described is suitable for grid pulsing of tube 3. In the class of triodes havin closely spaced electrodes and adapted to produce high frequency oscillations, it is simpler to produce a squar pulse of low voltage for modulating the grid than it is to produce a square pulse of high voltage for modulating the plate.

Since tubes 3 and 23 operate from a common alternating current source, the anodes 4 and 24 have a positive potential applied simultaneously so that they are each in an operative condition at the same time.

To control the time of firing of tube 23, the phase shifting circuit described above delays the voltage applied to grid 25 relative to the voltage applied to the anode 24. In Fig. 2, there is shown the relative phase relationship of the voltage applied to anodes 4 and 24, and the voltage applied to the grid 25. The waveform at a-in Fig. 2 is that applied to the anodes 4 and 24 while the waveform at b is that applied to grid 25. As may be seen from Fig. 2, the alternating voltage 3 applied to grid 25 lags that applied to anode 24 by a few degrees.

By proper orientation of coil 32, the phase shifting device is adjusted so that tube 23 will fire at the time indicated by a point C on curve b in Fig. 2. The pulse from resistor 39 formed by the discharge of pulse-forming network 3.] is applied to grid of 'tube 3. This will occur when anode 4 is positive. Since grid 5 will be negative at this time (due to the bias from potentiometer M), the pulse must overcome. the ne ative bias before tube 3 will oscillate.

Since the modulating pulses have a duration in the order of magnitude of a microsecond, the use of unrectified alternating current on the anode of tube 3 is entirely satisfactory. If the transmitter is caused to oscillate during each cycle at a point C in Fig. 2, there will be no appreciable change in plate voltage during the microsecond pulse at the grid of the tube.

In the copending application Serial. No. 584,236, filed March 22, 1945, by Howard L. Schultz and Harold G. Cooper, there is disclosed a radio receiver which will operate on. alternating current voltages without rectification. The receiver de: scribed in that application may be used with the transmitter herein described in a radio direction and range determining system.

The receiver described in the above mentioned application is sensitive for consecutive, limited periods of time. With the phase-shifting circuit adjusted so that tube'23 will fire at a point slightly before the peak of the applied sine wave, as indicated at a point C in b of Fig. 2, the. pulse transmitted by the transmitter herein described will occur at the right time with respect to the sensitive period of the receiver. The receiver then becomes sensitive for a limited period of time after the transmission of the pulse in order to receive reflected signals.

What we claim to have invented is:

1. A pulse modulated radio transmitter .-sys+ tem comprising a source of alternating voltage,

'an oscillatory circuit including a vacuum tube having at least a plate, a grid and a cathode,.said

grid and said plate being connected to opposite ends of said voltage source whereby said oscillatory circuit is normally rendered inoperative by mutually opposing voltages, a modulator circuit including a gaseous tube having at least a plate, -a grid and a cathode, the modulator tube being normally non-conductive, a phase shifting network. coupled to themodulator'grid, means for applying a portion ofsaid voltage source to the modulator tube plate whereby the voltage applied thereto is in phase with the voltage applied to the plate of the oscillator tube, means for applying said same voltage portion to said modulator grid through said phase shifting network to control the time for periodically rendering said modulator tube conductive whereby an outputpulse is periodically generated by said modulator circuit, means for controlling the duration of said pulse, and means for applying said pulse to the grid of the oscillator tube whereby said oscillatory circuit is rendered operative only for the duration of said pulses.

2; A pulse'modulated radio transmitter system comprising a source of alternating voltage, an oscillatory circuit including a vacuum tube having at least a plate, a grid and a cathode, said grid and, said plate being connected to opposite ends of said voltage source whereby said oscillatory circuit is normally rendered inoperative by mutually opposing voltages, a modulator circuit including a gas tube having at least a plate, a grid and a cathode, said gas tube :being normally non-conductive, a'phase shifting network coupled to the modulator grid, means for applying a portion of said voltage to the. plate of the gas tube whereby the voltage applied thereto is in phase with the voltage applied to the plate of the oscillator tube, means ioriapplying said same voltage portion to .said modulator grid through said phase shifting network to control the time of the periodic discharge of said gas tube thus generating a modulatoroutput pulse, means for controlling the duration of :said modulator pulse, and means for applying said pulse to the grid of said oscillator tube whereby said oscillatory circuit is rendered operative only for the duration of the pulse.

'3.A transmitter system as claimed in :claim 1, wherein the phase shifting network includes a fixed coil having a portion of said alternating voltage applied thereto, a movable coil, and means for varying the axis of said movable .coil in relation to the :axis of said fixed coil.

HAROLDJS. Gr. COOPER. HOWARD L. SCHULTZ.

REFERENCES CITED The following references are of record in the file of this, patent:

UNITED STATES PATENTS Number Name .Date

1,563,342 Conrad Dec. .1, 1925 1,566,245 Brackett Dec. 15,1925 

